Variable-length transmission line



June 30, 1953 M. c. PEASE 2,644,140

VARIABLE-LENGTH TRANSMISSION LINE Filed Oct. 19, 1945 2 Sheets-Sheet 1 L l' I 5 2 I V I; g k 2 i 82 Q Q I Q i E A n I INVENTOR. MARSHALLCZPEASE ATTORNEY Patented June 30, 1953 VARIABLE-LENGTH TRANSMISSION LINE Marshall 0. Pease, Boston, Mass, assignor' to the United States of America as represented 'by the Secretary of War Application. October 19, 1945, SerialNo..623,417

3 Claims.

This invention relates in general to coaxial lines for transmitting energy at radio frequencies. More particularly it relates to a lineof adjustable length so constructed that it has only minor mechanical and electrical discontinuities and thus introduces minimum reflections into the line and the associated apparatus.

In radio frequency technique at frequencies where transmission lines are used as reactance elements as well as for the purpose of transmitting energy, it is frequently desirable to have a section of line in which the standing waves are controllable. In the past this has been done by varying the characteristic impedance of the line, by the use of stubs, shorting plungers, and reactance elements, and in numerous other ways. Such devices have serious disadvantages because they vary both the position of the standing waves on the line, that is, their phase, and the standing wave ratio. It is frequently desired to vary these two characteristics independently. To vary the standing wave phase without varying the ratio, all that is needed is to vary the length of the line but, up to the present, all methods of doing this have created discontinuities which, as above explained, change the standing wave ratio.

It is an object of the present invention to provide a coaxial transmission line of adjustable length which introduces a minimum of reflections into the line.

A furtherobject of this invention is to provide a. coaxial transmission line that will alter the phase of the standing wave condition Of the line without materially affecting the standing wave ratio.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and Will become apparent from the following description of the invention taken in connection with the accompanying drawings in which Fig. l is a longitudinal cross-section of a coaxial transmission line constructed in accordance with the principles of this invention;

Fig. 2 is a longitudinal cross-section of the outer telescoping portion of the inner conductor of such a line;

Fig. 3 is a cross-section on line 33 of Fig. 2;

Fig. 4 is a longitudinal cross-section of the inner telescoping portion of the inner conductor of such a line; and

Fig. 5 is a cross-section on line 55 of Fig. 4.

Referring in particular to Fig. l, which shows an assembled section of line fitted in end connectors, outer telescoping outer conductor I0 and inner telescoping outer conductor II are connected at their opposite ends to suitable conventional connectors l2,, l2 each having dielectric discs l3, l3 forsupporting the two portions l4 and I5 of the inner conductor.

Outer portion l0 of the outer conductor is a plain tubular member of proper diameter to fit snugly around inner portion 1!. Inner portion II at the end fitting into outer portion I0 is formed with tapered contact fingers IS; The taper'on these fingers should extend at least a large-fraction of the wavelength corresponding tothe lowest operating frequency at which the line will be operated. These fingers taper from the thickness of the solid part of the inner portion l l to a knifeedge thickness and are sprung slightly outward to'mak'e-good contact with outer portion l0; For this reason the material of which inner portion ll is'made must have resiliency as well as conductivity; This spring effect of fingers l6 also givesmechamcal rigidity to the structure. At the other end of inner portion 1 l where it is connected-to connector l2 through sleeve N, there is a tapered portion I8 which corresponds in length to that of the taper on fingers I6.

Outer telescoping portion M of the inner conductor is held in place in connector 12 by dielectric supporting disc-13'. By reference to Figs. 2 and 3 it-may b'eseen-that portion 14 of the inner conductor comprises a metal tube with tongues or fingers l9 tapered at each of their ends. The slots between these tongues are also tapered at their ends. As may be seen from Figs; 4 and 5, the other or inner telescoping portion l5 of the inner conductor is solid but has cut in its periphery grooves or slots 20 into which tongues I9 fit when the section of line is assembled. Grooves 20 are tapered at each of their ends in order to aid in keeping the parts in line and to add general rigidity to their structure, tongues 19 are bevelled so they are much wider at their bases than at the circumference and grooves 20 are undercut to fit the shape of tongues l9.

By again referring to Fig. 1, it may be seen that the assembly of portions l4 and 15 of the inner conductor forms a rod which is extendable and which has substantially the same circumference along its length regardless of whether it is in a closed, extended, or intermediate position. The only deviation from a constant circumference is to be found at the portions of grooves 20 which are not filled by tongues 19 when the section is in an extended position. It has been found, although when in a partially or fully extended position the rod is not a true circle, that electrically for all material purposes it acts as if it Were such provided grooves 20 are not too wide. The efiect of any remaining physical difference is minimized because of the tapers at the ends of grooves 29, on the ends of tongues 19, and at the ends of the slots between those tongues. Since the Width of the inner portion of the outer conductor is very small compared to the inner diameter of the outer conductor and since its end is tapered it presents a minimum change in characteristic impedance where outer portion and inner portion I! meet. Taper 18 reduces the discontinuity because of the difference in diameter of portions i9 and 1!. As is well known, the characteristic impedance of a coaxial line is dependent upon the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor. It may therefore, be seen that there is very little change in characteristic impedance along the section of a line as described above. Therefore, When the length of the section is changed it is possible to change the standing wave phase without making any material change in the standing wave ratio.

For best operation the section shouldbe connected so that inner portion H of the outer conductor is on the load side of the system and outer portion 19 is on the generator side. In this way,

any reflections that do occur in the section of 1 line will add to those in the load and be compensated for by using matching devices for that purpose, whereas if load and generator were connected the other way around, such compensation would be lost each time the length of the sectio is varied.

It will be obvious to those skilled in the art that many different details of construction may be used while retaining the principles of this invention. For example, the type of connectors used and the manner of connecting the outer conductor to them and of supporting the inner conductor within the outer conductor is no part of this invention.

While there has been here described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is: 1. In a coaxial transmission line for radio frequency energy, means for varying the length of said line without introducing material electrical discontinuities including an inner conductor having a hollow portion with relatively long tongues tapered at their ends and disposed about its'circumference and another portion with relatively long grooves tapered at their ends and disposed about its circumference within which grooves said tongues fit, make electrical contact, and are adapted to slide, and an outer conductor having a longitudinally and internally tapered first portion with resilient fingers and a corresponding internal but longitudinally opposite taper at the other end, said fingers bein adapted to telescope Within and to make contact with a second portion of said outer conductor.

2. In a coaxial transmission line for radio fre: quency energy, means for varying the length of said line without introducing material electrical discontinuities including an inner conductor having a hollow portion with relatively long tongues tapered at their ends and disposed about its circumference and anotherportion with relatively long grooves tapered at their ends and disposed about its circumference within which grooves said tongues fit, make electrical contact, and are adapted to slide, and an outer conductor having a first portion with longitudinally and internally tapered resilient fingers adapted to telescope within and to make contact with a second portion of said outer conductor.

3. A variable-length coaxial transmission line for radio frequency energy, including an inner and. outer conduct-or, said inner conductor comprising a pair of end-to-end slidably-connected portions, one portion having relatively long tapered tongues disposed about its periphery, and the other portion having relatively long tapered grooves disposed about its periphery Within which grooves said tongues fit, make electrical contact, and are adapted to slide, said outer conductor having a first portion slidably engaged with a second portion.

' MARSHALL C. PEASE.

References Cited in the file of this patent UNITED STATES PATENTS Hewitt Dec. 9, 1947 

